Nixie-clock using neon lamps as logic elements

The above shows my home-built digital clock.
It uses Nixie-tubes
for readout.
In contrast to most other nixie-clocks being built these days,
my clock does not use any transistor or IC for driving the tubes.
Instead, the driving logic is built from
neon lamps, together with
resistors, capacitors and silicon diodes.

The project started in 2002, when
our university library was
selling old outdated or otherwise superfluous books, and I very cheaply
bought the book "Electronic Counting Circuits"
by J.B. Dance, published in 1967, and apparently only ever lent
three times by our library, all in 1973.
It described how neon lamps can be used as logic elements in a ring counter,
exploiting the fact that they need
a higher voltage to ignite (the striking voltage)
than to stay lit (the maintaining voltage):
Unfortunately, if one substitutes the neon bulbs that are available in
electronics shops nowadays, the circuit doesn't work.
Dance used lamps that were specifically manufactured for this type
of application, with a large difference between their
striking and maintaining voltages.
Nowadays, such lamps are (presumably) no longer manufactured;
the neon bulbs that are still available in shops are meant as indicator
lamps, and have a much smaller difference between their striking and
maintaining voltages.
This required changing the circuit's resistor values, and makes its
operation more critical; furthermore, the lamps need to be selected
for matching characteristics.

This is one of the ring counters in my clock:
Four of these are used, to divide the 50 Hz from the mains power
(see here for stability measurements)
first by 10 (yielding 5 Hz), then by 5 (yielding 1 Hz, i.e.,
one pulse per second), then further by 10 and 6 to yield
one pulse per minute.
Note the paper labels still dangling at the cathode wires of the
lamps: these are needed to look up the measured properties
of each lamp.

Four more ring counters are used dividing by 10, 6, 10 and 3, to count
the minutes, tens-of-minutes, hours and tens-of-hours
and drive the Nixie tubes:
The nixie tubes are driven through Light Dependent Resistors (LDRs):
under the influence of the light from the neon lamp, their resistance
lowers, connecting one nixie cathode to the negative power supply.
In order for the LDR not to be influenced too much by ambient light, while
still allowing the neon bulb to be visible, an optical attenuator
and filter is used between them, consisting of a black cardboard disk
with a small hole in it, and two layers of
red foil,
held together by glue and shrink tube:

The ring counters are rather sensitive to ambient light: in complete darkness,
they tend not to work. Even though there are always a few bulbs active (if only
in the power supply, which is not shown in the photographs), my clock
still needs a bit of external ambient light.
I'm experimenting with blue LEDs for providing this extra ambient light.
This seems to be quite effective: illuminated by just two blue leds,
the clock ran perfectly one night long in otherwise complete darkness:
Note though that the blue in this photo is more intense than it
looks like in reality:
apparently the camera is more sensitive to this shade of blue
than the human eye.

Some other things that I ran into while designing this clock:

In contrast to what Dance's book says, one can't cascade the
ring counters just by connecting them (when using modern-day neon bulbs).
I'm now using an extra neon bulb per counter as an amplifier:
it is biased to just under its striking voltage, so a small pulse
can strike it.

The striking and maintaining voltages of the lamps
change quite much during their first hours of operation.
Therefore, it is necessary to first "burn-in" (age) the
lamps, before measuring their characteristics.

Despite selecting my lamps for matching characteristics, some still acted weird
and needed to be replaced. For example, I had one which somehow didn't work
reliably in the buffer stage; and another one worked reliably in a ring counter
when clocked at about 1 Hz, but not when getting a pulse only once per hour.
Apparently, fully characterizing the neon lamps requires more quantities than
just the striking and maintaining voltages.

The clock is now electrically functional, but still some work remains to be
done. The power supply needs to be built tidily, the aligator clip test leads
eliminated, and the whole thing put into a (transparent) enclosure for safety.

Movie and circuit diagram

A short movie (AVI format, 10 MB) of the clock in operation
is available here.

Furthermore, the circuit diagram is available in a
PDF file.
This schematic diagram contains some extra explanation of how specific
parts work.
This diagram is meant to document and explain the details of my clock,
and there will probably be some minor changes made in the future.
The diagram is not meant as a complete basis for building another
such clock; for example, while some of the resistor values are quite
uncritical and determined by what I happened to have at hand,
many depend critically on the characteristics of the neon lamps used.
(Hopefully needless to say, any prospective builders should take proper safety
precautions for working with the high voltages involved.)

Links

Many people these days enjoy building Nixie clocks, though usually with
modern electronics driving the tubes.
See here
for a gallery.

Nixie clocks completely without silicon have also been built:
this one
and
this one
use vacuum tubes,
and this one
uses trigger tubes. Trigger tubes are actually neon lamps with a
third electrode to trigger them.

In the November 1966 issue of Electronics Illustrated,
a description was published for building an electronic calculator
using neon bulbs connected as ring counters.
(I've also seen reference to a 1967 issue of
"Practical Electronics" as apparently
containing the same or a very similar article.)